This invention relates to a hydraulic lock with an override capability which allows for normal operation when placed in either tension or compression, but prevents the lock from being stroked in tension by dynamic forces at or above a predetermined velocity.
Although the present invention has wider applications, it will be herein described with reference to a leg rest for a seat of the type presently found on many airliners. The lock includes a hydraulic cylinder having a piston contained therein that is secured to a piston rod that passes out of the cylinder through one end wall. The rod is attached to the seat frame while the cylinder is attached to the leg rest. The lock is placed in tension as the rest moves from a generally vertical stowed position to a raised elevated position and in compression when the rest is moved back into the stowed position. A flow channel is provided in the piston that permits fluid to flow between a front and a rear chamber in the cylinder when the lock is stroked. A normally closed valve that can be activated manually by a seated passenger controls the flow of fluid through the piston. A set spring is used to bias the valve in the closed position and can be overridden without manually activating the system by simply pulling the rest upwardly at a relatively low velocity. A compression spring is employed which becomes fully compressed when the rest is stowed and which unloads to help elevate the rest when the lock is stroked in tension.
The lock override feature works well in practice to prevent damage to the system in the event a passenger who is not familiar with manual activation procedure attempts to raise the rest by physically pulling the rest out of the stowed position. The override has one drawback, however, in that it is velocity or acceleration sensitive. Accordingly, leg rest in unoccupied seats can be inadvertently raised when the lock is loaded dynamically in tension. Once raised, the leg rests will remain elevated until such time as they are manually returned to a stowed position. In the event of an emergency, where high dynamic forces are generated, many of the rests may be inadvertently raised, thereby creating a safety hazard and making it difficult for people to move about the cabin.